In many electronic circuit applications, multiple power supplies are connected in parallel to drive a common load during different times of operation. One application example is a device that implements a standby or “sleep” mode. During standby mode such a device might use a low power DC supply such as a battery or DC-DC converter to power the minimal circuitry required to “awaken” the device, and upon awakening switch to a higher power DC supply that supports the current requirements of the functional circuitry.
In switched power supply systems, switching devices are used to switch different power supplies to actively provide power to a common load. These switching devices are controlled using dedicated control logic that only allows one voltage source to supply power to the common load. In many applications, the load is sensitive to large voltage deviations. Accordingly, it is important to limit the voltage deviation seen at the load even when the source of power is being switched from one power supply to another.
In voltage deviation sensitive loads, the implementation choice of the switching devices becomes important. Switching relays switch too slowly to meet strict voltage deviation limitation requirements when used alone. The switching performance can be improved with the use of very large capacitors; however, this increase the expense and size of the overall system.
Analog switches are also a poor choice for voltage deviation sensitive loads. Analog switches are characterized by a high internal resistance, which can create a voltage drop at the load greater than the allowed voltage deviation during normal operation.
Recently, N-Channel MOSFETs are being used to switch between multiple different power supplies to actively power a common load. In such a switching arrangement, the MOSFETs are connected with their drains tied together at the load and their respective sources connected to the output of their respective power supplies.
As termed herein, when a MOSFET switch associated with a particular power supply is turned OFF to isolate its respective power supply from the load, the respective power supply is referred to as an “isolated power supply”. When the MOSFET switch is turned ON to connect its respective power supply to the load, the respective power supply is referred to herein as an “active power supply”. As will be appreciated by those skilled in the art, in a switched power supply system, all power supplies switchably connected to the load may remain powered ON; accordingly, although an isolated power supply is isolated from the load, it may still supply power at its output.
Due to its construction, an N-Channel MOSFET is characterized by an intrinsic body diode across the source and drain. In particular, the anode of the intrinsic body diode is connected at the source node and the cathode is connected at the drain node. In the MOSFET arrangement just described, wherein the drains of each switching MOSFET are tied together, the cathodes of the intrinsic body diodes in the MOSFETs are tied together. This design configuration creates the appearance of using OR-ing diodes. The voltage source outputs must be within a diode drop (approximately 0.6 volts) of each other because if the output voltage of an isolated power supply is greater than a diode drop of an active power supply, it will forward bias the intrinsic body diode in the isolated power supply's associated MOSFET switch and will also supply power to the load. Accordingly, unless the output voltages of each of the power supplies are within a diode drop of each other, their associated MOSFET switches will not provide isolation even if one MOSFET switch is on and the others are off. In particular, the power supply with an output voltage greater than a diode drop of another power supply will source current to the load even though its MOSFET switch is turned off by the forward bias created by the voltage differential across the intrinsic body diode of its switch.
Even if the output voltages of each switched power supply are within a diode drop of one another, a failure in the active power supply will cause a forward bias of the intrinsic body diode of the isolation switch of the isolated power supply, causing the isolated power supply to supply power directly into the failed power supply. The active power supply may then go into current limit. If the active power supply is allowed to continue to operate in current limit, it may eventually damage the MOSFET switch of the isolated power supply due to excessive power dissipation in its intrinsic body diode.
A need therefore exists for protecting the MOSFET isolation switches in a MOSFET switched power supply system when a failure occurs in one of the power supplies. A need also exists for protecting the remaining non faulty power supplies to ensure that the remaining power supplies, and therefore the load, remains within specified tolerance limits.